Low power bipolar transistors with low parasitic losses

ABSTRACT

Low power double heterojunction bipolar transistors with low parasitics, and methods for making the same is presented. A transistor is fabricated such that the collector region underneath a base contact area is deactivated. This results in a drastic reduction of the base-collector parasitic capacitance, C bc . An embodiment of the present invention provides a transistor architecture for which the base contact area can be decoupled from the collector and hence allows for dramatic reduction in the parasitics of transistors.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

This invention was made with U.S. Government support under Contract No. N66001-01-C08033 awarded by the Defense Advanced Research Projects Agency through the Space and Naval Warfare Systems Command (DARPA/SPAWAR). The U.S. Government has certain rights to this invention.

FIELD OF THE INVENTION

The invention relates generally to bipolar junction transistors (BJTs) and methods for making the same. More particularly, the invention relates to low power bipolar transistors with low parasitic losses and methods for making the same.

BACKGROUND OF THE INVENTION

Typically, a BJT consists of a vertical stack of npn or pnp semiconductor materials. A BJT can take the form of a double heterojunction bipolar transistor (DHBT), as compared to a homojunction BJT. High speed operation is achieved through judicious scaling down of the thickness of the vertical stack of semiconductors and scaling down of the lateral dimensions of the size and spacing of the contacts. The electron transit time decreases though device scaling of the vertical and lateral dimensions. Also, the parasitics associated with capacitance increases as the vertical dimensions are reduced. For example, thinning the base and collector layers reduces the carrier transit times but increases the base resistance and base collector capacitance. The base collector capacitance can be reduced through lithographically scaling the base, collector, and emitter regions. However, the parasitics associated with the contact resistance increase as the lateral dimensions are reduced, since resistance is proportional to the contact area.

Vertical scaling of the epitaxial structure and lithographic lateral scaling are the traditional approaches used to improve transistor performance. A cross-sectional view of a conventional Indium Phosphide based DHBT (InP-based DHBT) that utilizes the triple mesa architecture is illustrated in FIG. 1, which depicts the prior art. Here, emitter contact 10, base contacts 20, and collector contact 30 all lie at different vertical positions. There is a base-collector capacitance, C_(bc), which includes the base collector capacitance from the intrinsic base (base region located adjacent the emitter in the vertical direction) and the extrinsic base (the base region which extends laterally from the intrinsic base region. The extrinsic base includes the region to which external base contacts 20 are made). As the extrinsic base area is reduced, the capacitance C_(bc) decreases, but the base contact resistance increases. As such, the conventional mesa structure is not optimal for parasitics reduction due to the following disadvantage: Independent optimization of the extrinsic base contact area and the capacitance C_(bc) is not possible. Hence there is significant base-collector capacitance which arises from the collector region that lies under the base contact. Additionally, the base contact area cannot be increased without incurring a capacitance C_(bc) penalty.

In view of the forgoing, the above described traditional BJTs have a base contact area that is coupled to a parasitic base-collector capacitance C_(bc), thereby frustrating device scaling. As such, there is a continuing need to reduce the parasitic capacitance and the base resistance of BJTs, thereby enabling operation of the BJTs at higher speed. The present invention addresses this continuing need as well.

SUMMARY OF THE INVENTION

The invention relates to a low power double heterojunction bipolar transistor (DHBT), and methods for making the same. The low power heterojunction bipolar transistor includes a collector, a base, and a emitter. The collector, the base, and the emitter are made of Indium-Arsenide-Phosphide or InAs_(1−x)P_(x) (x=0 to 0.5). The presence of a narrow-gap semiconductor, such as InAs (InAs_(1−x)P_(x) (x=0)) whose band gap is 0.35 eV in a portion of the base layer, enables low power operation. The invention provides a transistor embodiment that is fabricated such that the collector region underneath a base contact area is deactivated. This results in a drastic reduction of the base-collector parasitic capacitance, C_(bc).

One exemplary embodiment of the present invention provides a bipolar transistor that has an extrinsic base and a collector. The embodiment reduces a parasitic base-collector capacitance of the bipolar transistor by deactivating a part of the collector region that overlaps the extrinsic base.

Deactivation of a part of the collector is performed by selective type-converting the collector region that is adjacent the extrinsic base of the bipolar transistor. The type-converted collector region can be included as part of the extrinsic base. The extrinsic base can extend partially into a type converted buffer layer or a substrate or both substrate and buffer layer that is located adjacent the type converted collector region. In addition, a reduction in both a parasitic base contact resistance and a base access resistance is also achieved through the use of selective doping for fabricating large and deep extrinsic base contact regions.

The extrinsic base can be doped heavier than an intrinsic base of the bipolar transistor, and an area of the extrinsic base can be increased without incurring a base-collector capacitance penalty.

In a conventional transistor, the sub-collector extends under the entire collector, as shown in FIG. 1. In the present invention, the sub-collector is located laterally from the emitter and extrinsic base. A sub-collector of the bipolar transistor can be formed using a selective doping technique by masking regions that are not doped. The selective doping technique can include an ion implantation technique, a dopant diffusion technique or a damage assisted dopant diffusion technique. A damage of the damage assisted dopant diffusion technique can be caused by ion-implantation.

The extrinsic base can be formed using a selective doping technique. The selective doping technique can include an ion implantation technique, a dopant diffusion technique, or a damage assisted dopant diffusion technique. A damage for the damage assisted dopant diffusion technique can be caused by ion-implantation. Since InAs is prone to lattice damage from ion implantation, the addition of phosphorus to InAs results in the strengthening of the lattice, thereby facilitating recovery of the implant related lattice damage during the anneal process. Since residual implant damage in InAs may make it difficult to achieve a high level of p-type conductivity, phosphorus is added to facilitate damage recovery.

The bipolar transistor can also include suitable passivation scheme to shield the collector from electrical breakdown.

The collector, the intrinsic base, the extrinsic base, and the emitter of the bipolar transistor are made of InAs_(1−x)P_(x) (x=0 to 0.5)

One exemplary embodiment of the present invention provides a bipolar transistor for reducing a parasitic base-collector capacitance. The bipolar transistor includes a collector, an extrinsic base, an intrinsic base, and an emitter. The collector is adjacent to a substrate. In the present exemplary embodiment, the parasitic base-collector capacitance C_(bc) is reduced by deactivating a part of the collector that overlaps the extrinsic base.

One exemplary embodiment of the present invention provides a bipolar transistor that includes: a plurality of semiconductor layers having a collector layer, a base layer in contact with the collector layer, and an emitter layer in contact with the base layer, the base layer having the opposite doping type of the collector layer and the emitter layer, wherein the plurality of semiconductor layers are formed to include: an intrinsic base region adjacent to and in contact with the emitter layer and an extrinsic base region, the extrinsic base region being adapted to make contact with a base electrode and extending laterally from the intrinsic base region; and a sub-collector region connected to a main collector region, located laterally from the main collector region, and adapted to make contact with a collector electrode, the sub-collector region being doped heavier than the collector layer, and wherein a region of the collector layer adjacent to the extrinsic base region is selectively type converted to have the same doping type as the base layer. The base layer, the collector layer, and the emitter layer include substantially of InAs_(1−x)P_(x), wherein x ranges from 0 to 0.5.

Also, in one embodiment, the plurality of semiconductor layers further include a support layer in contact with the collector layer, a region of the support layer adjacent to the type converted region of the collector layer is also selectively doped with the same doping type as the base layer, and the support layer can include a buffer layer.

In one embodiment, the type converted region of the collector layer and the selectively doped region of the support layer become a part of the extrinsic base region.

In one embodiment, the type converted region of the collector layer becomes a part of the extrinsic base region.

In one embodiment, the sub-collector region is formed by selectively doping a portion of the collector layer, and the sub-collector region is substantially located laterally from the intrinsic base region.

In one embodiment, the plurality of semiconductor layers further include a support layer in contact with the collector layer, the sub-collector region is formed by selectively doping a portion of the collector layer and a portion of the support layer, the sub-collector region is substantially located laterally from the intrinsic base region, and the support layer can include a buffer layer.

In one embodiment, the bipolar transistor further includes an emitter formed by the emitter layer, wherein a lateral region of the collector layer between the base region and the sub-collector region is passivated to improve breakdown characteristics.

In one embodiment, a heterojunction is formed between the emitter layer which includes InAs_(1−x)P_(x), wherein x ranges from 0 to 0.5, and the base layer which includes InAs_(1−y)P_(y), wherein y is not greater than x.

In one embodiment, a first heterojunction is formed between the emitter layer which includes InAs_(1−x)P_(x), wherein x ranges from 0 to 0.5, and the base layer which includes InAs_(1−y)P_(y) (y<x), wherein y is not greater than x, and a second heterojunction is formed between the base layer and the collector layer which includes InAs_(1−x)P_(x), wherein x ranges from 0 to 0.5. Compositions of the collector, base and emitter layers can be graded in the compositions to facilitate the unimpeded flow of carriers.

In one embodiment, the base electrode contacting the extrinsic base region can be increased in area without significantly increasing the base-collector capacitance.

In one embodiment, the composition of the collector layer is substantially InAs_(1−x)P_(x), (x=0.20) and a part of the collector layer is graded from x=0.20 to x=0 in the vicinity of the base layer. The composition of the base layer is graded from x=0 at a junction between the base layer and the collector layer to x=0.10 at a junction between the base layer and the emitter layer. The composition of a part of the emitter layer a vicinity of the base layer is graded from x=0.25 at the junction between the base layer and the emitter layer to x=0.20. The composition of the rest of the emitter layer has x=0.20.

In one embodiment, the extrinsic base region is selectively doped heavier than the intrinsic base region.

In one embodiment, each of the extrinsic base region and the sub-collector region is formed by a masking method and an implant method.

In one embodiment, each of the extrinsic base region and the sub-collector region is formed by a masking method and a diffusion method.

In one embodiment, each of the extrinsic base region and the sub-collector region is formed by a masking method and an implant and diffusion method, the implant and diffusion method includes an implant method followed by a diffusion method, and the implant method provides damages for enhancing diffusion in a vertical direction.

In one embodiment, the plurality of semiconductor layers are formed to become a more planar transistor adapted to improve heat dissipation.

In one embodiment, the plurality of semiconductor layers are formed to become an active component in an integrated circuit.

In one embodiment, the sub-collector region is implanted with silicon (Si).

In one embodiment, the extrinsic base region is doped with zinc (Zn), beryllium (Be), or both Zn and Be.

In one embodiment, the sub-collector region is implanted with silicon (Si), and the extrinsic base region is doped with zinc (Zn) and beryllium (Be).

In one embodiment, the extrinsic base region and the sub-collector region are formed with the emitter layer intact.

In one embodiment, the extrinsic base region and the sub-collector region are formed by etching away a part of the emitter layer.

In one embodiment, the sub-collector region is formed with the base layer intact.

In one embodiment, the sub-collector region is formed by etching away a part of the base layer.

A more complete understanding of the bipolar transistors with low parasitic losses and the methods for making the same will be afforded to those skilled in the art, as well as a realization of additional advantages and objects thereof, by a consideration of the following detailed description. Reference will be made to the appended sheets of drawings which will first be described briefly.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, together with the specification, illustrate exemplary embodiments of the present invention, and, together with the description, serve to explain the principles of the present invention, wherein:

FIG. 1 is a schematic diagram of a traditional InP-based DHBT;

FIGS. 2A, 2B, and 2C are schematic diagrams illustrating a process flow for fabricating a BJT according to an embodiment of the present invention;

FIGS. 3A, 3B, and 3C are schematic diagrams illustrating a process flow for fabricating a BJT according to an alternative embodiment of the present invention;

FIG. 4 is a schematic diagram of an exemplary DHBT according to the present invention.

DETAILED DESCRIPTION

The invention is described below, with reference to detailed illustrative embodiments. It will be apparent that the invention can be embodied in a wide variety of forms, some of which may be quite different from those of the disclosed embodiments. Consequently, the specific structural and functional details disclosed herein are merely representative and do not limit the scope of the invention.

The invention relates to bipolar junction transistors (BJTs) and heterojunction bipolar transistors (HBTs) and methods for making the same. HBTs with architectures that enable the independent optimization of extrinsic base resistance and base-collector capacitance C_(bc) are highly desirable. An embodiment of the present invention provides a transistor architecture for which the base contact area can be decoupled from the collector and hence allows for dramatic reduction in the parasitics of transistors. A transistor in one embodiment of the present invention uses a selectively doped sub-collector and base contact regions, and can be fabricated such that there is no active collector region underneath the base contact area as shown in FIG. 2C. This device offers several advantages over conventional bipolar transistors, such as:

-   -   1) The deactivation of collector region under the extrinsic base         through selective doping of the extrinsic base and type         conversion of the collector results in a drastic reduction in         the base-collector capacitance C_(bc). The base contact area is         therefore decoupled from the active collector region.     -   2) Lower extrinsic base contact resistance R_(bex) is achieved         because of three favorable factors:         -   i) reduction of the base contact resistance through higher             doping in the extrinsic base;         -   ii) use of larger base contact area without incurring a             base-collector capacitance penalty, and         -   iii) the thick extrinsic base contact region results in             reduced base access resistance.

Referring to FIGS. 2A, 2B, and 2C, a dramatic reduction of base-collector capacitance C_(bc) is achieved by a transistor architecture wherein the selectively type converted collector region 155 under the extrinsic base does not contribute significantly to the base-collector capacitance C_(bc). As shown in FIG. 2B, beryllium (Be) implantation and/or zinc (Zn) diffusion is employed for the type conversion and deactivation of a part of the collector (n−InAs_(1−x)P_(X)) layer (or material) 102 under base layer (p+InAs_(1−x)P_(X)) 103. The type converted collector 155 becomes a part of the extrinsic base which includes the original extrinsic base region 150. This result in a thick extrinsic base (which includes 150 and 155) which has reduced extrinsic base resistance R_(bex), and the freedom to utilize large base contacts 160 without incurring a base-collector capacitance penalty. Sub-collector 110 is located adjacent to the collector 130, in a lateral direction (as opposed to the triple mesa structure which has a vertical orientation) and is formed by silicon (Si) implantation, as shown in FIG. 2B, for producing n+InAsP.

As formed, the DHBT of FIG. 2C is more planar as compared to the standard triple mesa structure, due to the lateral positioning of sub-collector 110. Since the collector is in direct physical contact with substrate and buffer layer 170, the DHBT of FIG. 2C should have improved local heat dissipation characteristics, which is an important attribute for transistors operating at high current densities.

In addition, a buffer layer can be included as being part of support layer 101 with a substrate. That is, some device structures use a buffer layer grown atop the substrate (e.g., substrate and buffer layer 170) before the growth of collector, base, and emitter layers. However, the present invention is not thereby limited and various modifications appreciated by those skilled in the art may be made.

Furthermore, the collector, base, and emitter layers can also be graded in phosphorus content to facilitate an unimpeded carrier flow.

In one embodiment, the composition of the collector layer is substantially InAs_(1−x)P_(x), (x=0.20) and a part of the collector layer is graded from x=0.20 to x=0 in the vicinity of the base layer. The composition of the base layer is graded from x=0 at a junction between the base layer and the collector layer to x=0.10 at a junction between the base layer and the emitter layer. The composition of a part of the emitter layer at a vicinity of the base layer is graded from x=0.25 at the junction between the base layer and the emitter layer to x=0.20. The composition of the rest of the emitter layer has x=0.20.

In more detail, FIGS. 2A, 2B, and 2C illustrate a process flow for fabricating a DHBT according to an embodiment of the present invention. FIG. 2A shows epitaxial structures prior to implantation. The epitaxial structures include a sequence of semiconductor layers having support layer 101, which can include a buffer layer, collector layer 102, base layer 103 in contact with collector layer 102, and emitter layer 104 in contact with base layer 103. FIG. 2B shows the structures undergoing the sub-collector and extrinsic base implants. FIG. 2C shows the BJT in which the region 155 under extrinsic base 150 does not contribute to base-collector capacitance C_(bc) due to the selective type conversion of region 155 into a portion of extrinsic base.

FIGS. 3A, 3B, and 3C illustrate a process flow for fabricating a BJT according to another alternative embodiment of the present invention. FIG. 3A shows epitaxial structure prior to implantation with a defined or etched emitter 310. FIG. 3B shows the structures undergoing the sub-collector and extrinsic base implants. FIG. 3C shows the BJT in which the type converted region 300 under the original extrinsic base 350 does not contribute significantly to the base-collector capacitance C_(bc).

In general, an exemplary device architecture of the present invention provides a drastically lower parasitic capacitance and base resistance. Additionally, an exemplary transistor of the present invention is fabricated using ion-implantation, with implant-damage assisted impurity diffusion. As such, embodiments of the present invention provide a drastic departure from layouts and architectures used in traditional bipolar transistors.

In particular, referring now back to FIGS. 2A, 2B, and 2C, the present invention provides a bipolar transistor embodiment that includes a sequence of semiconductor layers having support layer 101, collector layer 102, base layer 103 in contact with the graded collector, layer 102, and emitter layer 104 in contact with base layer 103. Base layer 103 has a doping type that is the opposite of collector layer 102 and emitter layer 104 (or emitter 120). Intrinsic base region 140 contacts emitter 120, and extrinsic base region 150. Extrinsic base region 150 makes contact with base contact metal (or electrode) 160.

More specifically, extrinsic base region 150 extends laterally (or horizontally) from the intrinsic base region 140 that lies directly adjacent to (or vertically below) emitter 120. Electrical contact to the entire base is made though base contact metal (or electrode) 160 contacting extrinsic base 150.

In addition, a substantial portion of sub-collector 110 is located laterally (or horizontally) from collector 130. Sub-collector 110 can be formed with base layer 103 intact or by etching away a part of base layer 103.

Collector region 100 adjacent to (or vertically) below extrinsic base 150 is selectively type-converted to the same doping type as the base to become a portion of extrinsic base 150. That is, extrinsic base 150 includes the type-converted collector region 100. The selective type conversion can be achieved through ion-implantation, dopant diffusion, ion-implant damage assisted dopant diffusion, by opening windows where doping is required and masking other regions where selective doping is not required.

As such, base and access resistance of an BJT embodiment of the present invention is reduced because of the presence of a thick base which includes the type converted collector region 100. In addition, the base contact resistance can in one embodiment be further decreased by more heavily doping extrinsic base 150 as compared to that in intrinsic base 140. Moreover, the base-collector capacitance C_(bc) from extrinsic base 150 is significantly (or substantially) less than the base collector capacitance C_(bc) when compared to that of a conventional BJT. The area of extrinsic base 150 can be increased without a significant increase in the base collector capacitance C_(bc).

Referring to FIG. 4, an exemplary BJT (e.g., DHBT) of the present invention includes lateral sub-collector 750. Specifically, the exemplary BJT includes emitter 710 that resides on top of intrinsic base 720. The intrinsic base 720 resides on top of collector 730. Collector 730 is formed on top of substrate and buffer layer 740. The extrinsic base includes the original extrinsic base 760 and the type converted collector region 765. The sub-collector 750 that is located laterally from the collector 730, is formed by selectively doping a part of the collector. Moreover, passivation layer (or isolation layer) 770 (e.g., an oxide or nitride) is placed on top of collector 730 not covered by intrinsic base 720 to shield or isolate collector 730 from any possible electrical breakdown (i.e., passivation layer 770 is used to protect the collector region between emitter 710 and sub-collector 750). As such, the exemplary BJT of FIG. 4 decouples the area contacting a metal contact of extrinsic base 760 from collector 730 and reduces the base-collector capacitance by minimizing the overlap of extrinsic base 760 with the active collector 730. In addition, the exemplary BJT can improve thermal dissipation of the BJT due to the more planar shape of the BJT, since the collector is directly in contact with the substrate and buffer layer 740. Moreover, integrated circuits may include these exemplary BJTs and other passive components.

In FIG. 4, both extrinsic base 760 and sub-collector 750 are selectively doped and may extend to (or partially into) substrate and buffer layer 740. Ion-implantation can be used to realize the selectively doped lateral sub-collector 750. As such, sub-collector 750 is formed by selective doping of the collector and substrate materials. The selective doping of sub-collector 750 can be achieved through ion-implantation, dopant diffusion, or ion-implant damage assisted dopant diffusion.

Ion-implantation can also be used to make extrinsic base 760 that may extend into substrate and buffer layer 740. That is, extrinsic base 760 may include the type converted collector and the type converted substrate and buffer layer 740. The selective type conversion of the extrinsic base 760 can be achieved through ion-implantation, dopant diffusion, and/or ion-implant damage assisted dopant diffusion.

In certain exemplary embodiments of the present invention, the collector and emitter can be made of InAs_(1−x)P_(x) (x=0 to 0.5), and the base can be made of InAs_(1−y)P_(y) (y<x).

In view of the foregoing, embodiments of the present invention provide a BJT that decouples the base contact area (area of the base below the base contact metal) from the influence of the collector, resulting in a reduction of C_(bc). Specifically, the present invention provides a transistor embodiment that has a transistor architecture and layout that has no active collector region underneath a base contact area. In addition, the invention provides a method for fabricating a transistor such that, after fabrication, the collector region underneath the base contact area of the transistor is deactivated. As such, embodiments of the invention result in a drastic reduction of the base-collector parasitic capacitance, C_(bc).

In addition, certain transistor embodiments of the present invention use selectively doped sub-collectors and base contact regions and are fabricated such that there is no active collector region underneath a base contact area. This approach offers several advantages as compared to the conventional bipolar transistors. For example, the base contact area is decoupled from the collector. The deactivation of the collector material under the base contact results in a drastic reduction in the base-collector capacitance, C_(bc). The extrinsic base can be doped heavier than the intrinsic base, resulting in reduction of both a base contact resistance and a base access resistance.

While certain exemplary embodiments have been described in detail and shown in the accompanying drawings, it is to be understood that such embodiments are merely illustrative of, and not restrictive of the broad invention. It will thus be recognized that various modifications may be made to the illustrated and other embodiments of the invention described above, without departing from the broad inventive scope thereof. 

1. A bipolar transistor comprising: a plurality of semiconductor layers comprising a collector layer, a base layer in contact with the collector layer, a support layer in contact with the collector layer, and an emitter layer in contact with the base layer, the base layer having the opposite doping type of the collector layer and the emitter layer, wherein the plurality of semiconductor layers are formed to comprise: an intrinsic base region adjacent to and in contact with the emitter layer and an extrinsic base region, the extrinsic base region being adapted to make contact with a base electrode and extending laterally from the intrinsic base region; and a sub-collector region connected to a main collector region, located laterally from the main collector region and not interposed between the main collector region and the support layer to thereby increase transistor speed of the bipolar transistor, and adapted to make contact with a collector electrode, the sub-collector region being doped heavier than the collector layer, wherein a region of the collector layer adjacent to the extrinsic base region is selectively type converted to have the same doping type as the base layer, wherein the base layer, the emitter layer, and the collector layer comprise substantially of InAs_(1−x)P_(x), wherein x ranges from 0 to 0.5, wherein a region of the support layer adjacent to the type converted region of the collector layer is also selectively doped with the same doping type as the base layer, and wherein the sub-collector region is not interposed between the extrinsic base region and the support layer.
 2. The bipolar transistor of claim 1, wherein the support layer comprises a buffer layer.
 3. The bipolar transistor of claim 2, wherein the type converted region of the collector layer and the selectively doped region of the support layer become a part of the extrinsic base region.
 4. The bipolar transistor of claim 1, wherein the type converted region of the collector layer becomes a part of the extrinsic base region.
 5. The bipolar transistor of claim 1, wherein the sub-collector region is formed by selectively doping a portion of the collector layer, and wherein the sub-collector region is substantially located laterally from the intrinsic base region.
 6. The bipolar transistor of claim 1, wherein the sub-collector region is formed by selectively doping a portion of the collector layer and a portion of the support layer, wherein the sub-collector region is substantially located laterally from the intrinsic base region, and wherein the support layer comprises a buffer layer.
 7. The bipolar transistor of claim 1, wherein a part of the collector layer comprises InAs_(1−x)P_(x) graded from x=0.20 to x=0 at a vicinity of the base layer, wherein other parts of the collector layer comprises substantially of InAs_(1−x)P_(x) having x=0.20, wherein the base layer comprises InAs_(1−x)P_(x) graded from x=0 at the junction between the base layer and the collector layer to x=0.10 at the junction between the base layer and the emitter layer, wherein a part of the emitter layer at a vicinity of the base layer comprises InAs_(1−x)P_(x) graded from x=0.25 at the junction between the base layer and the emitter layer to x=0.20, and wherein other parts of the emitter layer comprises substantially of InAs_(1−x)P_(x) having x=0.20.
 8. The bipolar transistor of claim 1, further comprising an emitter formed by the emitter layer, wherein a lateral region of the collector layer between the emitter and the sub-collector region is passivated to improve breakdown characteristics.
 9. The bipolar transistor of claim 1, wherein a first heterojunction is formed between the emitter layer and the base layer and a second heterojunction is formed between the base layer and the collector layer.
 10. The bipolar transistor of claim 1, wherein the compositions of the collector layer, the base layer, and the emitter layer can be graded in phosphorus content.
 11. The bipolar transistor of claim 1, wherein the emitter layer and the collector comprise substantially of InAs_(1−x)P_(x), wherein x ranges from 0 to 0.5, wherein the base comprises substantially of InAs_(1−y)P_(y), and wherein y is not greater than x.
 12. The bipolar transistor of claim 1, wherein the base electrode contacting the extrinsic base region can be increased in area without significantly increasing the base-collector capacitance.
 13. The bipolar transistor of claim 1, wherein the extrinsic base region is selectively doped heavier than the intrinsic base region.
 14. The bipolar transistor of claim 1, wherein each of the extrinsic base region and the sub-collector region is formed by a masking method and an implant method.
 15. The bipolar transistor of claim 1, wherein each of the extrinsic base region and the sub-collector region is formed by a masking method and a diffusion method.
 16. The bipolar transistor of claim 1, wherein each of the extrinsic base region and the sub-collector region is formed by a masking method and an implant and diffusion method, wherein the implant and diffusion method comprises an implant method followed by a diffusion method, and wherein the implant method provides damages for enhancing diffusion in a vertical direction.
 17. The bipolar transistor of claim 1, wherein the base layer, the emitter layer, and the collector layer include phosphorus (P) in the InAs lattice to facilitate recovery of the implant damage during an annealing phase to thereby minimize residual conductivity from implant damage.
 18. The bipolar transistor of claim 1, wherein the plurality of semiconductor layers are formed to become a more planar transistor adapted to improve heat dissipation.
 19. The bipolar transistor of claim 1, wherein the plurality of semiconductor layers are formed to become an active component in an integrated circuit.
 20. The bipolar transistor of claim 1, wherein the sub-collector region is implanted with silicon (Si).
 21. The bipolar transistor of claim 1, wherein the extrinsic base region is doped with zinc (Zn), beryllium (Be), or both Zn and Be.
 22. The bipolar transistor of claim 1, wherein the extrinsic base region and the sub-collector region are formed with the emitter layer intact.
 23. The bipolar transistor of claim 1, wherein the extrinsic base region and the sub-collector region are formed by etching away a part of the emitter layer.
 24. The bipolar transistor of claim 1, wherein the sub-collector region is formed with the base layer intact.
 25. The bipolar transistor of claim 1, wherein the sub-collector region is formed by etching away a part of the base layer.
 26. A bipolar transistor comprising: a plurality of semiconductor layers comprising a collector layer, a base layer in contact with the collector layer, a support layer in contact with the collector layer, and an emitter layer in contact with the base layer, the base layer having the opposite doping type of the collector layer and the emitter layer, wherein the plurality of semiconductor layers are formed to comprise: an intrinsic base region adjacent to and in contact with the emitter layer and an extrinsic base region, the extrinsic base region being adapted to make contact with a base electrode and extending laterally from the intrinsic base region; and a sub-collector region connected to a main collector region, located laterally from the main collector region, and adapted to make contact with a collector electrode, the sub-collector region being doped heavier than the collector layer, wherein a region of the collector layer adjacent to the extrinsic base region is selectively type converted to have the same doping type as the base layer, wherein the base layer, the emitter layer, and the collector layer comprise substantially of InAs_(1−x)P_(x), wherein x ranges from 0 to 0.5, wherein the sub-collector region is not interposed between the extrinsic base region and the support layer, wherein a region of the support layer adjacent to the type converted region of the collector layer is also selectively doped with the same doping type as the base layer, and wherein the sub-collector region is formed by selectively doping a portion of the collector layer and a portion of the support layer.
 27. A bipolar transistor comprising: a plurality of semiconductor layers comprising a collector layer, a base layer in contact with the collector layer, a support layer in contact with the collector layer, and an emitter layer in contact with the base layer, the base layer having the opposite doping type of the collector layer and the emitter layer, wherein the plurality of semiconductor layers are formed to comprise: an intrinsic base region adjacent to and in contact with the emitter layer and an extrinsic base region, the extrinsic base region being adapted to make contact with a base electrode and extending laterally from the intrinsic base region; and a sub-collector region connected to a main collector region, located laterally from the main collector region, and adapted to make contact with a collector electrode, the sub-collector region being doped heavier than the collector layer, wherein a region of the collector layer adjacent to the extrinsic base region is selectively type converted to have the same doping type as the base layer, wherein the base layer, the emitter layer, and the collector layer comprise substantially of InAs_(1−x)P_(x), wherein x ranges from 0 to 0.5, wherein the sub-collector region is not interposed between the extrinsic base region and the support layer, wherein the sub-collector region is also not interposed between the intrinsic base region and the support layer, wherein a region of the support layer adjacent to the type converted region of the collector layer is also selectively doped with the same doping type as the base layer, and wherein the sub-collector region is formed by selectively doping a portion of the collector layer and a portion of the support layer. 